Conventionally, a ball-point pen refill comprising an ink storage cylinder having a thermochromic ink loaded therein is well known, for example, a writing instrument containing a thermochromic ink is disclosed in Japanese Unexamined Patent Application Publication No. 2011-153321 “Reversible thermal color changing aqueous ink composition and writing instrument using the same”.
The handwriting formed using a ball-point pen refill, which comprises an ink storage cylinder having such a thermochromic ink loaded therein, can be decolored by heating. Consequently, as has been widely recognized the ball-point pen refill is used for a writing instrument for taking notes and making marks as a substitute for pencils and mechanical pencils.
Typically, in a writing instrument, in order to improve the visibility of handwriting, the handwriting density is also preferably enhanced. Moreover, it is believed that, when copying papers having written notes etc. thereon, using e.g., a monochrome copy machine, the handwriting density is preferably enhanced in order to bring about a sufficient copying density.